The ability to ensure a consistently high quality of manufactured semiconductor components, for example semiconductor wafers and dies, is increasingly crucial in the semiconductor industry. Semiconductor wafer fabrication techniques have been consistently improved for incorporating an increasing number of features into a smaller surface area of the semiconductor wafer. Accordingly, the photolithographic processes used for semiconductor wafer fabrication have become more sophisticated to allow the incorporation of increasing number of features into the smaller surface area of the semiconductor wafer (i.e. for achieving higher performance of the semiconductor wafers). Consequently, sizes of potential defects on semiconductor wafers are now typically in the micron to submicron range.
It is evident that manufacturers of semiconductor wafers have an increasingly pressing need to improve semiconductor wafer quality control and inspection procedures to ensure a consistently high quality of manufactured semiconductor wafers. Semiconductor wafers are typically inspected for detecting defects thereon, such as presence of surface particulates, imperfections, undulations and other irregularities. Such defects could affect eventual performance of the semiconductor wafer. Therefore, it is critical to eliminate or extract defective semiconductor wafers during the manufacturing of semiconductor wafers.
There have been advances in semiconductor inspection systems and processes. For example, higher resolution imaging systems, faster computers, and enhanced precision mechanical handling systems have been commissioned. In addition, semiconductor wafer inspection systems, methods and techniques have historically utilized at least one of brightfield illumination, darkfield illumination and spatial filtering techniques.
With brightfield imaging, small particles on the semiconductor wafer scatter light away from a collecting aperture of an image capture device, thereby resulting in a reduction of returned energy to the image capture device. When the particle is small in comparison with the optical point spread function of a lens or digitalizing pixel, brightfield energy from the immediate areas surrounding the particle generally contribute a large amount of energy relative to the particle, thereby making the particle difficult to detect. In addition, the very small reduction in energy due to the small particle size is often masked by reflectivity variations from the immediate areas around the particle thereby resulting in increased occurrences of false defect detection. To overcome the above phenomena, semiconductor inspection systems have been equipped with high-end cameras with larger resolutions, which capture images of smaller surface areas of the semiconductor wafer. Brightfield images generally have a better pixel contrast and this is advantageous for estimating size of defects and when inspecting dark defects.
Darkfield imaging and its advantages are generally well-known in the art. Darkfield imaging has been employed with several existing semiconductor wafer inspection systems. Darkfield imaging typically depends on the angle at which light rays are incident on the object to be inspected. At a low angle to a horizontal plane of the object to be inspected (for example 3 to 30 degrees), darkfield imaging typically produces a dark image except at locations where defects, such as surface particulates, imperfections and other irregularities exist. A particular use of darkfield imaging is to light up defects which sizes are smaller than the resolving power of lens used to produce brightfield images. At a higher angle to the horizontal plane (for example 30 to 85 degrees), darkfield imaging typically produces better contrast images as compared to brightfield images. A particular use of such high angle darkfield imaging enhances contrast of surface irregularities on a mirror finish or transparent object. In addition, high angle darkfield imaging enhances imaging of tilted objects.
Light reflectivity of the semiconductor wafer typically has a significant effect on quality of images obtained with each of brightfield and darkfield imaging. Both micro and macro structures present on the semiconductor wafer affect the light reflectivity of the semiconductor wafers. Generally, amount of light reflected by the semiconductor wafer is a function of the direction or angle of incident light, the viewing direction and the light reflectivity of the surface of the semiconductor wafer. The light reflectivity is in turn dependent on wavelength of the incident light and material composition of the semiconductor wafer.
It is generally difficult to control the light reflectivity of semiconductor wafers presented for inspection. This is because the semiconductor wafer may consist of several layers of material. Each layer of material may transmit different wavelengths of light differently, for example at different speeds. In addition, layers may have different light permeabilities, or even reflectivity. Accordingly, it will be apparent to a person skilled in the art that the use of light or illumination of a single wavelength or a narrow band of wavelengths typically adversely affects quality of captured images. Need for frequent modification of the single wavelength or narrow band of wavelengths requires use of multiple spatial filters or wavelength tuners, which is generally inconvenient. To alleviate such problems, it is important to use a broadband illumination (i.e. illumination of a broad or wide range of wavelengths), for example broadband illumination of a range of wavelengths between 300 nm and 1000 nm.
Broadband illumination is important for achieving high quality images as well as for inspecting semiconductor wafers with a wide range of surface reflectivities. In addition, defect detection capabilities of wafer inspection systems will generally be enhanced by use of multiple illumination angles or contrasts, for example use of both brightfield and darkfield illuminations. Existing wafer systems in the market typically do not utilize illuminations of multiple angles and with a full broadband wavelength.
Currently available wafer inspection systems or equipments typically use one of the following methods for achieving multiple responses during wafer inspection:
(1) Multiple Image Capture Devices with Multiple Illuminations (MICD)
The MICD uses a plurality of image capture devices and a plurality of illuminations. The MICD is based on the principle of segmenting the wavelength spectrum into narrow bands, and allocating each segmented wavelength spectrum to individual illuminations. During design of systems employing the MICD method, each image capture device is paired with a corresponding illumination (i.e. illumination source), together with corresponding optical accessories such as a spatial filter or a specially coated beam splitter. For example, wavelength of the brightfield illumination is limited between 400 to 600 nm using a mercury arc lamp and a spatial filter and the wavelength of the darkfield illumination is limited between 650 to 700 nm using lasers. The MICD method experiences disadvantages, for example inferior image quality and a relative inflexibility in system design or configuration. The inferior image quality is generally due to varying surface reflectivities of inspected semiconductor wafers, combined with the use of illuminations with narrow wavelengths for inspecting the semiconductor wafers. Design inflexibility of the system occurs because the modification of the wavelength of a single illumination used with the system typically requires reconfiguration of the entire optical setup of the system. In addition, the MICD method typically does not easily enable capture of illuminations of varying wavelengths by a single image capture device without compromising the quality of captured images or the speed of capturing images.
(2) Single Image Capture Device with Multiple Illuminations (SICD)
The SICD method uses a single image capture device for capturing multiple illuminations, each of the multiple illuminations being either of segmented wavelengths (i.e. narrow band of wavelengths) or of broadband wavelengths. However, it is not possible to obtain multiple illumination responses simultaneously while the semiconductor wafer is in motion. In other words, the SICD method only allows one illumination response when the semiconductor wafer is in motion. To achieve multiple illumination responses, the SICD method requires capture of images while the semiconductor wafer is stationary, which affects throughput of the wafer inspection system.
Semiconductor wafer inspection systems employing simultaneous, independent, on-the-fly image capture using broadband brightfield illumination and darkfield illumination, or in general multiple illuminations, and using multiple image capture devices are not presently available due to a relative lack of understanding as to actual implementation and operating advantages thereof.
As described above, existing semiconductor wafer inspection systems typically employ either MICD or SICD. Equipments employing MICD do not use broadband illuminations and typically suffer from inferior image quality and inflexibility in system setup or configuration. On the other hand, semiconductor wafer inspection systems using SICD experience diminished system throughput and are incapable of obtaining on-the-fly simultaneous multiple illumination responses.
An exemplary existing semiconductor wafer optical inspection system that utilizes both brightfield illumination and darkfield illuminator is disclosed in U.S. Pat. No. 5,822,055 (KLA1). An embodiment of the optical inspection system disclosed in KLA1 utilizes MICD as described above. The optical inspection system disclosed in KLA1 uses multiple cameras to capture separate brightfield and darkfield images of semiconductor wafers. Captured brightfield and darkfield images are then processed separately or together for detecting defects on the semiconductor wafers. In addition, the optical inspection system of KLA1 captures brightfield and darkfield images simultaneously using separate sources of brightfield and darkfield illumination. The optical inspection system of KLA1 achieves simultaneous image capture (i.e. capture of brightfield and darkfield images) by using illumination emitters emitting illuminations of segmented wavelength spectrums and spatial filters. With the optical inspection system of KLA1, one of the cameras is configured to capture darkfield images with corresponding use of a narrow band laser illumination and spatial filter. Another camera is configured to capture brightfield images with corresponding use of brightfield illumination and a beam splitter having a special coating. Disadvantages of the optical inspection system disclosed by KLA1 include unsuitability thereof for imaging semiconductor wafers comprising a large variation of surface reflectivities. This is due to use of illuminations of segmented wavelength spectrums. The cameras are each for capturing illumination of a predetermined wavelength spectrum. There is little flexibility for each camera to capture illuminations of multiple different wavelength spectrums for enhancing captured images of certain wafer types. For example, wafers comprising a carbon-coated layer on their first surface exhibit poor reflection characteristics at certain illumination angles, for example with brightfield illumination. Accordingly, a combination of brightfield illumination and high angle darkfield illumination is required for viewing certain defects on such wafers. The optical inspection system of KLA1 utilizes a plurality of illumination emitters or sources and filters. The optical inspection system of KLA1 performs multiple inspection passes (i.e. multiple scans) to thereby enable the capture of both brightfield and darkfield images. Consequently, the throughput of the optical inspection system is adversely affected.
Additional exemplary existing optical inspection systems utilizing both brightfield and darkfield imaging are disclosed in U.S. Pat. No. 6,826,298 (AUGTECH1) and U.S. Pat. No. 6,937,753 (AUGTECH2). The optical inspection systems of AUGTECH1 and AUGTECH2 utilize a plurality of lasers for performing low angle darkfield imaging, and a fiber optic ring light for performing high angle darkfield imaging. In addition, the optical inspection systems of AUGTECH1 and AUGTECH2 each utilizes a single camera sensor and the SICD method as explained earlier. Accordingly, inspection of semiconductor wafers by the optical inspection systems of AUGTECH1 and AUGTECH2 is performed either by brightfield imaging or by darkfield imaging or via a combination of both brightfield imaging and darkfield imaging wherein each of the brightfield imaging and darkfield imaging is performed when the other is completed. The inspection system of AUGTECH1 and AUGTECH2 is not capable of simultaneous, on-the-fly, and independent brightfield and darkfield imaging. Accordingly, multiple passes of each semiconductor wafer are required for completing inspection thereof. This results in lowered manufacturing throughput and an increased utilization of resources.
In addition, several existing optical inspection systems utilize a golden image or a reference image for comparison with newly acquired images of semiconductor wafers. Derivation of the reference image typically involves capturing several images of known or manually selected “good” semiconductor wafers, and then applying a statistical formula or technique to thereby derive the reference image. A disadvantage with the above derivation technique is inaccuracies or inconsistencies associated with manual selection of the “good” semiconductor wafers. Optical inspection systems using such reference images can suffer from false rejects of semiconductor wafers due to inaccurate or inconsistent reference images. With increasingly complex circuit geometry of semiconductor wafers, the reliance on manual selection of “good” semiconductor wafers for deriving reference images is becoming increasingly incompatible, particularly with the increasing quality standards set by the semiconductor inspection industry.
Deriving a golden reference image involves many statistical techniques and calculations. Most of existing statistical techniques are very general and have their own merits. Currently available optical inspection systems or equipment typically use either average or mean together with standard deviation when deriving a golden reference pixel. Use of mean with standard deviation for deriving golden reference pixels can be useful with known good pixels; otherwise any defect or noise pixel would interfere and affect final average or mean value of the reference pixel. Another statistical technique utilizes median for reducing interference due to noise pixel. However, it is not possible, or at least difficult, to substantially eliminate the effect of noise. Existing optical inspection systems or equipment try to reduce the effect of noise by applying varying statistical techniques. However, a user friendly or simple method for reducing or eliminating the effect of noise (i.e. error) has yet to be devised. Such a method will help to eliminate noise pixels, which would affect the final reference pixel value.
U.S. Pat. No. 6,324,298 (AUGTECH3) discloses a training method for creating a golden reference or reference image for use in semiconductor wafer inspection. The method disclosed in AUGTECH3 requires “Known Good Quality” or “Defect Free” wafers. Selection of such “Known Good Quality” wafers is manually or user performed. Statistical formulas or techniques are then applied for deriving the reference image. As such, accurate and consistent selection of “Known Good Quality” wafers is crucial for maintaining a high quality of semiconductor inspection. The method of AUGTECH3 uses mean and standard deviation to calculate individual pixels of the reference image. Accordingly, presence of any defective pixel will lead to inaccurate derivation of reference pixel. The defective pixel can occur due to foreign matter or other defects. Such foreign matter or defects could adversely affect the statistical calculation and lead to inaccurate derivation of reference pixel. It will be apparent to a person skilled in the art that the method of AUGTECH3 is open to inaccuracies, inconsistencies and errors in inspection of the semiconductor wafers.
In addition, optical inspection system disclosed in AUGTECH3 uses a flash or strobe lamp for illuminating the semiconductor wafers. It will be appreciated by a person skilled in the art that inconsistencies between different flashes or strobes may occur due to numerous factors including, but not limited to, temperature differentials, electronic inconsistencies and differential flash or strobe intensities. Such differentials and inconsistencies are inherent even with “good” semiconductor wafers. Presence of such differentials would affect the quality of derived golden reference images if the system does not take such differentials into consideration. In addition, illumination intensity and uniformity varies across the surface of the semiconductor wafer due to factors including, but not limited to, differing planarity of the wafer, mounting and light reflectivities at different positions on the surface of the semiconductor wafer. Without taking into account the above-mentioned differentials and factors, any reference images derived in the above-described manner may be unreliable and inaccurate when used for comparison with captured images at different positions on the surface of the semiconductor wafers.
Variations in product specifications, for example semiconductor wafer size, complexity, surface reflectivity, are common in the semiconductor industry. Accordingly, semiconductor wafer inspection systems and methods need to be capable of inspecting semiconductor wafers of different specifications. However, existing semiconductor wafer inspection systems and methods are generally incapable of satisfactorily inspecting semiconductor wafers of a wide range of different specifications, especially given the increasing quality standards set by the semiconductor industry.
For example, a typical existing semiconductor wafer inspection system uses a conventional optical assembly comprising components, for example cameras, illuminators, filters, polarizers, mirrors and lens, which have fixed spatial positions. Introduction or removal of components of the optical assembly generally requires rearrangement and redesign of the entire optical assembly. Accordingly, such semiconductor wafer inspection systems have inflexible designs or configurations, and require a relatively long lead-time for modification thereof. In addition, distance between objective lens of the convention optical assembly and semiconductor wafer presented for inspection is typically too short to allow ease of introduction of fiber optics illumination with differing angles for facilitating darkfield imaging.
There are numerous other existing semiconductor wafer inspection systems and methods. However, because of current lack of technical expertise and operational know-how, existing semiconductor wafer inspection systems cannot employ simultaneous brightfield and darkfield imaging for an inspection while the wafer is in motion, while still having design and configurationally flexible. There is also a need for semiconductor wafer inspection systems and methods for enabling resource-efficient, flexible, accurate and fast inspection of semiconductor wafers. This is especially given the increasing complexity of electrical circuitry of semiconductor wafers and the increasing quality standards of the semiconductor industry.